<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
          function  test(){
               this.a = function (){
                    console.log(123);
               }
              this.b =  function (){
                   this.a();
               }
          }
        var t =  new test();
          t.b();
</script>
</body>
</html>